The present invention relates to a process for preparing aqueous solutions of cationic di- and triarylmethane dyes from the aqueous dye solutions, which contain water-soluble organic solvents.
It is common knowledge to use nanofiltration, sometimes also referred to as ultrafiltration, to concentrate dye solutions and also remove salts therefrom, as described in EP-A-0 037 382, for example. This is frequently employed in the case of azo dyes, since their synthesis involves high salt levels.
DE-A-2 805 891 describes the removal of sodium chloride from anionic dyes by diafiltration and the conversion of the anionic dyes into dyes having amines as cations.
The production of liquid forms of cationic triarylmethane dyes continues to pose a number of problems, since this class of dyes, unlike traditional azo dyes, is synthesized in the presence of water-soluble organic solvents, frequently organic acids. Organic solvents, which are subsumed under volatile organic compounds (VOCs), are unwelcome in the liquid use forms of the dyes. The removal of a water-soluble organic solvent is frequently possible only together with the water, necessitating an energy-intensive isolation of the solids, which, moreover, represents an additional thermal stress for the dye.
It is an object of the present invention to provide a more economical process for preparing aqueous solutions of cationic di- and triarylmethane dyes.
We have found that this object is achieved by a process for preparing aqueous solutions of cationic di- and triarylmethane dyes by the aqueous dye solution, which contains organic solvents, being concentrated by nanofiltration and optionally diluted with water.
The process of the present invention makes it possible to remove water-soluble organic solvents. The term xe2x80x9csolventxe2x80x9d does not necessarily mean that this organic substance was a solvent for the dye or for a precursor thereof. It may similarly be the cosolvent or, in the case of carboxylic acids for example, be formed from the counterion of the dye, and rather encompasses the group subsumed under the term VOC.
By water-soluble are meant solvents whose solubility in water is not less than 50 g/kg.
Examples are:
carboxylic acids such as formic acid, acetic acid, propionic acid and lactic acid
alkanols such as methanol, ethanol, n-propanol, isopropanol, ethylene glycol, propylene glycol and butylglycol
ethers such as tetrahydrofuran, dioxane, dimethoxyethane or diethylene glycol dimethyl ether.
The process of the present invention may be used to remove just one solvent or mixtures of two or more solvents.
The process of the present invention makes it possible to prepare aqueous solutions of cationic di- and triarylmethane dyes and is not limited to specific substitution patterns.
Cationic dyes are to be understood as meaning in particular dyes of the general formula I 
where
R1, R2, R3 and R4 are independently C1-C8-alkyl with or without substitution and with or without interruption by from 1 to 3 oxygen atoms in ether function, phenyl or C1-C4-alkylphenyl,
R5 and R6 are independently hydrogen or methyl,
X is hydrogen, substituted or unsubstituted phenyl or substituted or unsubstituted naphthyl, and
Anxe2x8ax96 is the equivalent of an anion,
which are obtained by oxidizing the corresponding leuco compound of the formula II 
These dyes may have both straight-chain and branched alkyl chains, which may further be substituted, for example by hydroxyl, chlorine, cyano, phenyl or hydroxysulfonylphenyl.
Naphthyl groups may for example be substituted by amino, mono- or di(C1-C4)alkylamino, mono- or diphenylamino or hydroxysulfonyl.
Phenyl groups in the formula I may be substituted for example by methyl, chlorine, amino, mono- or di(C1-C4)alkylamino, mono- or diphenylamino, hydroxyl, C1-C4-alkoxy or hydroxysulfonyl.
In substituted alkyl, phenyl or naphthyl groups, the number of substituents is in general from one to three.
Suitable anions include for example fluoride, chloride, bromide, iodide, hydrogensulfate, sulfate, tetrafluoroborate, formate, acetate, propionate, mono-, di- or trichloroacetate, lactate, methoxyacetate, citrate, succinate, methylsulfonate, benzenesulfonate or 2- or 4-methylbenzenesulfonate.
When the dyes of the formula I have hydroxysulfonyl radicals and are present in the salt form, suitable counterions are metal or ammonium ions. Metal ions are especially lithium, sodium or potassium ions. Ammonium ions for the purposes of the present invention are either unsubstituted or substituted ammonium cations.
Dyes of the formula I are common knowledge. Examples are:
malachite green (C.I. 42000 Basic Green 4),
crystal violet (C.I. 42555 Basic Violet 3),
methyl violet (C.I. 42535 Basic Violet 1),
Michler""s Hydrol Blue,
brilliant green (C.I. 42040 Basic Green 1),
Victoria Pure Blue (C.I. 42595 Basic Blue 7),
ethyl violet (C.I. 42600 Basic Violet 4) and
Victoria Blue B (C.I. 44045 Basic Blue 26).
Membranes useful for the process of the present invention are conventional membranes which are also useful for removing salts. These membranes are organic or inorganic porous membranes whose pores are from 1 to 500 xc3x85 in diameter. They are advantageously made of organic material which contains ionic groups. Of particular advantage are membranes having a molecular weight cut-off level of from 200 to 1000.
To force the permeate through the membrane, the pressures employed range from 5 to 50 bar, preferably from 15 to 25 bar. The membranes generally have permeate flux rates of up to 150 l/hm2.
Examples of suitable membrane materials are polysulfone, polyether sulfone, sintered metal, cellulose, cellulose acetate, polyamide, aramid, polyether, polyether sulfone, polytetrafluoroethylene, polyvinylidene fluoride or ceramics.
The membranes can have various forms, for example plate form, sheet form, tube form, pocket form, cone form or the form of hollow fibers. To resist high pressures, the membranes may of course be supported by wire screens or foraminous plates. Within the abovementioned range, the pore size may be varied by various heat treatments and likewise be optimized to the particular intended use.
Such membranes are commercially available and described in EP-A-0 037 382, for example. Such membrane separation processes, furthermore, have been described in detail, for example in H. Stratmann, H. Chmiel, Chem.-Ing.-Techn. 57 (1985) 581-596 and W. Pusch, A. Walch, Angew. Chem. 94 (1992) 670-695.
If desired or necessary, the dye solution is diluted with water and then concentrated by nanofiltration through removal of the permeate, which is a mixture of water and the water-soluble organic solvent. The amount of water to be added for dilution is relatively freely chooseable and merely limited by the solubility of the dye. The degree of dilution must not be so small as to cause the dye to precipitate during the nanofiltration. In general, the dye solution is diluted to a dye content of from 5 to 30% by weight. The nanofiltration removes up to 100 l of liquid per m2 of filter area per hour, depending on membrane type. This permeate is preferably replaced with water. The replacement of the permeate with water may take place both a little at a time and continuously.
In a preferred variant, which involves the dye solution being recirculated and the permeate being continuously replaced with water, the amount of water-soluble organic solvents is reduced to 1% by weight of their original level while the total amount of permeate removed is equal to 1-10 times the amount of recirculating dye solution.
The process of the present invention is particularly useful for removing carboxylic acids, especially acetic acid.
The process of the present invention is particularly effective at pHxe2x89xa64, preferably pH 1-3. More particularly, a pHxe2x89xa63 is advantageous for removing carboxylic acids.
Water-soluble organic solvents are frequently present in the reaction mixture because of the nature of the dye synthesis. For example, the dyes are synthesized with oxygen or hydrogen peroxide in the presence of a carboxylic acid such as acetic acid and a catalyst and therefore contain the carboxylic acid. In a preferred variant of the process, the reaction mixtures obtained by oxidation are, without intermediary isolation of the dye, admixed with a stronger acid than the carboxylic acid to be removed, the mixture is optionally clarified, optionally diluted with water, and nanofiltered, the last two measures being possibly repeated one or more times. The clarification is advantageously carried out after the diluting step.
A stronger acid has a lower pKa. Strong acids include both inorganic and organic acids. Preference is given to using acids whose anions form readily water-soluble salts with triarylmethane dyes, since concentrated dye solutions are usually desired.
Stronger acids useful for the removal of carboxylic acids include for example sulfuric acid, hydrochloric acid, methanesulfonic acid, orthophosphoric acid and amidosulfonic acid. The acid is generally used in an at least equinormal amount to the dye. If the pH of xe2x89xa63 was not reached, it is advantageous to add further acid.
The process of the present invention makes it possible to use as-synthesized dye solutions and to remove water-soluble organic solvents from them without costly intermediary isolation of the dye. The aqueous dye solutions thus obtained are stable and directly useful as liquid dyes.
The examples hereinbelow illustrate the process of the present invention.
The tests described in Examples 1-6 were carried out with the aid of a cross flow filtration unit equipped with a stack plate module. The module was fitted with 10 flat sheet membranes and had an effective filter area of 0.044 m2 in total. The requisite operating pressure and the cross flow was generated using a piston membrane pump. A pressure control valve was used to set the pressure drop across the membrane to 20 bar.